Solar cells are photovoltaic (PV) devices that convert light into electrical energy. Photovoltaic devices have been developed as clean, renewable energy sources to meet growing energy demand. Photovoltaic devices are being developed for a wide number of commercial markets including residential rooftops, commercial rooftops, utility-scale PV projects, building integrated PV (BIPV), building applied PV (BAPV) applications and the like. Widespread adoption of PV technology has not yet arrived, due in part to the high cost per watt for PV devices, particularly when compared to traditional electrical utility costs.
Currently, crystalline silicon based solar cells or photovoltaic devices (single crystal, multicrystalline and polycrystalline) are the dominant technologies in the market. Crystalline silicon (cSi) solar cells must use a thick substrate (>100 um) of silicon to absorb the sunlight since it has an indirect band gap. Also, the absorption coefficient is low for crystalline silicon because of the indirect band gap. The use of a thick substrate also means that the crystalline silicon solar cells must use high quality material to provide long carrier lifetimes to allow the carriers to diffuse to the contacts. Therefore, crystalline silicon solar cell technologies lead to increased costs.
Thin film photovoltaic (TFPV) devices have received increased interest as a replacement to crystalline silicon based PV devices. A variety of TFPV devices have been developed, such as TFPV devices based on amorphous silicon (a-Si), copper indium gallium diselenide (CIGS), and cadmium telluride (CdTe). Among these thin film technologies, some have already been very successful commercially and achieved much lower cost per watt than conventional Si-based PV. For example, CdTe-based thin film PV has demonstrated much lower cost and higher profitability than Si in recent years. CIGS based PV devices have garnered particular interest due to high demonstrated efficiencies when compared to the other TFPV materials.
Currently, CIGS layers used in PV devices are typically formed using vacuum based deposition processes where individual metal sources of copper, indium, gallium and selenium are evaporated towards a substrate in a vacuum chamber. Such vacuum based evaporation deposition processes are expensive, require high capital costs, and precise processing. Material utilization is poor, which further adds to the high manufacturing costs.
Co-evaporation of selenium onto a high temperature substrate in a high temperature environment causing many issues (e.g., Se corrosion, Se flux control) and is one of the largest challenges and bottlenecks in production of CIGS. Evaporation processes typically have a low material utilization rate and a limited material deposition rate, thus resulting in high raw material cost and low throughput. Process stability is another big issue for CIGS manufacturing using co-evaporation based technique. Achieving uniform film deposition across a large-area substrate is another significant challenge with currently known methods.
Another known selenization technique is carried out in a selenization furnace using a source of selenium such as H2Se. H2Se poses a significant safety risk even when diluted H2Se is used as the reactant. This increases the reaction time, which can be on the order of hours. Such furnaces are typically operated in batch mode, which significantly limits throughput. Moreover, many furnaces are needed to achieve desirable production volume, thus increasing capital and operating costs.
The manufacture of TFPV devices entails the integration and sequencing of many unit processing steps. As an example, TFPV manufacturing typically includes a series of processing steps such as cleaning, surface preparation, deposition, patterning, etching, thermal annealing, and other related unit processing steps. The precise sequencing and integration of the unit processing steps enables the formation of functional devices meeting desired performance metrics such as efficiency, power production, and reliability.
Thus, further developments are needed, particularly processes that lower the cost of manufacturing CIGS based PV devices and address some of the limitations of the current selenization techniques.